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A Decades Old Mystery Finally Solved

According to a letter written by Governor Leonard Calvert in May 1634, the colonists settled "within one half mile of the river, within a pallizado of one hundred and twentie yarde square, with four flankes" (Lee 1889, 3:21). The fort was also mentioned in a brief description included in a pamphlet based of the writings of Father Andrew White called A Relation of Maryland (Hall 1910).  

Based on these descriptions, Historic St. Mary’s City (HSMC) staff believed that St. Mary’s Fort was likely constructed in one of two locations: the ‘traditional site’ (south of Town Center, near the Print Shop) or a 15-acre tract known as Mill Field, which is further inland but lies along a creek that would have been deeper and more navigable by small vessels in the early 17th century. Early 17th-century artifacts had been found in both areas during previous archaeological investigations and both seemed promising.

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Map of HSMC showing the two sites targeted by the 2018 geophysical survey

Archaeology was conducted in the Mill Field beginning in the 1980s. In 1984–1985, HSMC conducted a controlled surface collection of most of the Mill Field. This entailed plowing the field, gridding it into 10′ squares, and systematically collecting and mapping artifacts found on the surface. This phase of work determined that there were numerous important sites in the Mill Field, including Native American, early 17th-century, and late 17th-century components (Neiman et al. 1984). Based on this work, HSMC's then-Chief Archaeologist Dr. Timothy Riordan (1991) hypothesized that the remains of St. Mary's Fort might lie in the Mill Field.

In 1992, with the support of a grant funded by the National Endowment for the Humanities and the hard work of students participating in the museum’s annual Field School in Historical Archaeology, HSMC visited the Mill Field again and excavated nearly 60 5×5′ units at the site. This work revealed a number of 17th-century artifacts and features, including evidence of blacksmithing and a robust palisade trench. However, the discovery and intensive study of the lead coffins buried in the Brick Chapel that year took precedence over further exploration. No further excavations were conducted in the Mill Field until 2019.

HSMC excavated portions of the “traditional site” in the mid-1990s. In 1993–1995, areas of Town Center were surface collected and investigated via a limited shovel test pit survey (Miller et al. 2006). This work resulted in the recovery of early 17th-century artifacts on a bluff adjacent to a ravine that leads into Key Swamp—an ideal spot for a military fortification. This area had long been speculated to be the possible location of St. Mary's Fort (Forman 1938).


As both the Mill Field and the “traditional site” held promising evidence, the true location of the fort remained a mystery. However, the question of which site was correct was forced to take the back seat for decades as HSMC worked on many other important findings and construction projects, such as the research, excavation, and reconstruction of Cordea’s Hope, William Smith’s Ordinary, the Van Sweringen Site Exhibit, the Print House, and the Brick Chapel. These efforts also included the design and installation of the St. John’s Archaeological Site Exhibit and the Struggle for Freedom Exhibit in the Brome Slave Quarter, as well as the planning and execution of major mitigation efforts in preparation for the construction of buildings on the campuses of both HSMC and St. Mary’s College of Maryland, including HSMC’s new laboratory and archival facilities in Anne Arundel Hall and the Maryland Heritage Interpretive Center, the museum’s new Visitor’s Center (construction scheduled to begin FY2022).

In 2017, Director of Research and Collections Dr. Travis Parno, decided to resume the search for St. Mary’s Fort. HSMC secured a Non-Capital Grant from the Maryland Historical Trust, with matching funds provided by the Historic St. Mary’s City Foundation for a new phase of exploration. The grant funds were used to hire expert geophysicist Dr. Timothy J. Horsley of Horsley Archaeological Prospection, LLC to conduct a geophysical survey of both the traditional site and the Mill Field. Dr. Horsley’s survey used three methodologies: magnetic susceptibility, magnetometry, and ground-penetrating radar.

Magnetic susceptibility measures the level of magnetism to which soils are susceptible, a factor that can be increased by activities associated with human habitation such as burning and disposal of organic waste. This type of survey can reveal where humans have or haven’t lived in the past, but cannot detect specific cultural features or provide dates of occupation. The magnetic susceptibility survey revealed that most of both of the target sites had been occupied by humans at some point, but it did little to narrow down the search.

Magnetometry measures contrasts in magnetic susceptibility, as well as the presence of permanent magnetization. Permanent magnetization occurs when material is heated to a high temperature. Magnetometry is useful for identifying cultural features such as pits, ditches, and areas of burning such as hearths or kilns. The magnetometry survey revealed little at the traditional site. In the Mill Field, the magnetometry identified several anomalies that were likely hearths or kilns, as well as multiple pits related to late 17th-century occupation.

Ground-penetrating radar (GPR) entails projecting radar waves into the ground and recording where the radar reflects back off of subsurface anomalies. When conducted at a high resolution, GPR can identify buried cultural features (e.g., foundations, trenches, hard-packed floors, post holes) and geological anomalies (e.g., rocks and boulders) with a high degree of accuracy. The GPR survey revealed little at the traditional site, but in the Mill Field, it revealed a large palisaded enclosure, along with hundreds of other cultural features both inside and outside of the enclosure.


Inside the palisaded enclosure, the GPR survey identified numerous post holes, pits, and other features. Inside the northwestern and northeastern walls, lines of post holes are suggestive of defensive architecture such as firing platforms. Inside the southern corner, an arc of posts may have been part of an interior bastion or firing platform. In the western half of the enclosure, there are areas where linear arrangements of post holes indicate substantial English timber frame buildings once stood. In the eastern half, multiple circular clusters of post holes are suggestive of Native American structures.

The GPR survey also identified multiple late 17th-century structures and landscape features (e.g. fence lines, road ditches). These buildings are oriented along the road to the Mill, a road that was part of the late 17th-century Baroque town plan of St. Mary’s City.


Using the results of Dr. Horsley’s survey, HSMC archaeologists, led by Dr. Parno, conducted excavations in the Mill Field and definitively located portions of the ditch used to construct the palisade that originally surrounded St. Mary’s Fort (archaeologists call the process of conducting excavations to test geophysical survey results “ground-truthing”). The excavations also confirmed the presence of the curved bastion on the fort’s western corner. 

Based on this new evidence, Dr. Parno and his team were able to both confirm the location and update the original rendering of what the fort contained. 

Now that the location is confirmed, HSMC will continue seasonal excavations in the Mill Field to learn more about the construction and internal arrangements of St. Mary’s Fort. The museum will also continue to evaluate options for interpreting the findings in the Mill Field. In the meantime, while excavations are being conducted, the dig site is open to visitors with a museum ticket. Please see our website for further details.

Visualization: A Changing Picture

St. Mary’s Fort was situated with Mill Creek to its north and Mattapany Road to the east; modern-day Mattapany Road is located on a path used for hundreds of years by Native Americans and colonial Marylanders. At the time of the fort’s construction, the Mill Field was likely cleared land. A Relation of Maryland describes the colonists “coming thus to seate upon an Indian Town, where they found ground cleered to their hands” (Hall 1910:76). 


Conceptual drawing of St. Mary’s Fort created by architectural historian Cary Carson in 1972 based on historical descriptions of the fort.

The first drawing of the St. Mary's Fort was created by Cary Carson in 1972 based on Calvert's 1634 description in his letter to Sir Richard Lechford. In his drawing, Carson skillfully depicted a massive fortification with four corner bastions, wide cannon emplacements, and defensive ditches outside the bastions. Life in the fort sprawls outside of the palisade walls to buildings erected near the main gate. In Carson's rendering, the fort is situated on a bluff overlooking the St. Mary's River.

However, based on the recent archaeological and geophysical survey findings, it appears that St. Mary’s Fort was rectangular (rather than square) and measured 312′ × 177′ (104 × 59 yards, or roughly 43% of the area specified by Governor Calvert’s letter). Why the difference between reality and Calvert's description? The answer may be found in other colonial records that suggested that although construction of St. Mary's Fort began in March of 1634, it may not have concluded until much later in the summer. When Governor Calvert sent his letter in late May, he may have been describing his plans for the fort, plans which were later altered as the summer wore on.


The fort’s exterior walls were palisades, or tall, sturdy wooden walls made by placing full or split timbers side-by-side in a deep trench. The timbers would have been secured by stringers, or horizontal boards or timbers attached to the vertical elements. The fort featured a circular bastion on its western corner and may have included internal firing platforms along portions of its other walls.

At this stage, we do not know exactly what the interior arrangement of the fort would have been. Historical records tell us that the colonists constructed a guardhouse and storehouse soon after landing. They also constructed dwellings, many of which were likely intended to be temporary residences while the colonists secured the colony and surveyed the property that would be become their permanent homes. Accounts from the early settlement suggest that colonial officials negotiated the rights to settle in a portion of the Yaocomaco village, so it is possible that the palisade encompassed Yaocomaco dwellings that served as temporary housing for the Anglo-Marylanders.

In the spring of 2020, Historic St. Mary's City solicited an updated sketch of St. Mary's Fort, one that would reflect what has been learned from recent research. The new rendering, created by Jeffrey R. Parno, depicts a rectangular fort with a single bastion. Inside the fort, a mixture of robust timber-frame structures, Yaocomaco wichotts (longhouses), and other types of temporary dwellings are visible. The fort is situated along a ravine amid land that had been cleared for agriculture. While artistic license was taken where archaeological data and educated guesswork could not supply precise details, the updated rendering demonstrates how our understanding of St. Mary's Fort has changed. As we continue excavations at the site of St. Mary's Fort, the visualization will continue to be refined.

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Conceptual sketch of St. Mary’s Fort created by Jeffrey R. Parno in 2020 based on recent geophysical survey and archaeological investigation


Forman, Henry C. 1938. Jamestown and St. Mary’s: Buried Cities of Romance. Baltimore: The Johns Hopkins Press.

Hall, Clayton C., editor. 1910. Narratives of Early Maryland 1633–1684. Barnes and Noble, New York, NY.

Lee, John Wesley Murray. 1848–1897. The Calvert Papers, No. 1–3. J. Murphy, Baltimore, MD.

Miller, Henry M., Ruth M. Mitchell, Timothy B. Riordan, Patricia Dance, James W. Embrey, Silas D. Hurry, Donald Winter,

          and Ilene J. Frank. 2006. “Archaeological Survey and Testing for the 17th-century ‘Mattapany Path’ Road System, St.

          Mary’s City, Maryland, 18ST1.” Report prepared for Maryland Department of Transportation, State Highway

          Administration, Baltimore, Maryland.

Neiman, Fraser D., Henry M. Miller, and Garry Wheeler Stone. 1984. “A Phase 1 Preconstruction Archaeological Survey of

          the Mill Field, Historic St. Maries Citty.” MS. on file Department of Research, HSMC.

Riordan, Timothy B. 1991. The Location of Fort St. Mary's: A Speculative Essay. Maryland Historical Magazine 86: 370–


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